The use of a stylus with a touch screen interface is well established. Touch screen designs have incorporated many different technologies including resistive, capacitive, inductive, and radio frequency sensing arrays. Resistive touch screens, for example, are passive devices well suited for use with a passive stylus. The original PalmPilots® devices from the mid-1990s were one of the first successful commercial devices to utilize a resistive touch screen designed for use with a stylus and helped to popularize that technology. Although resistive touch screens can sense the input from nearly any object, multi-touch is generally not supported. An example of a multi-touch application may be applying two or more fingers to the touch screen. Another example may be inputting a signature, which may include simultaneous palm and stylus input signals. Due to these and other numerous disadvantages, capacitive touch screens are increasingly replacing resistive touch screens in the consumer marketplace.
Various tethered active stylus approaches have been implemented for use with touch screens and are found in many consumer applications such as point-of-sale terminals (e.g., the signature pad used for credit card transactions in retail stores) and other public uses. However, the need for a tethered cable is a significant drawback for private applications such as personal computers (“PCs”), smart phones, and tablet PCs.
FIG. 1A is a block diagram illustrating a conventional embodiment of a host device 100 for tracking the position of a touch object on an inductive sense array 107. The host device 100 includes a printed circuit board (“PCB”) 105, a first matching circuit 110, a receiver 115, a host central processing unit (“CPU”) 120, a personal computer (“PC”) 125, a transmitter 130, and a second matching circuit 135. The PCB 105 is typically placed behind a touch screen (not shown) and includes an inductive sense array 107. The inductive sense array 107 includes a series of inductive coils. Inductive sense arrays are typically heavy and expensive to manufacture.
FIG. 1B is a block diagram illustrating a conventional embodiment of an active stylus 150 used in a system for tracking the position of a touch object on an inductive sense array 107. The stylus 150 includes a micro-controller unit (“MCU”) 155, a driver 160, and inductor eraser 165, and inductor tip 170, a rectifier 175, a power regulator 180, a button(s) 185, a force tip 190, and a force eraser 195. The inductor eraser 165 and inductor tip 170 are configured on different stylus edges.
In operation, the inductive sense array 107 on PCB 105 generates a magnetic field to provide both stylus power generation and touch position detection. Regarding touch position, the matching circuit 110 provides impedance matching and couples the stylus 150 signal from the inductive sense array 107 to the receiver 115. The receiver 115 and host CPU 120 receives and process the analog signal, respectively, providing touch position and force data to the PC 125. Force data is indicative of the amount of pressure provided by the stylus tip to the touch screen. The host CPU 120 calculates the touch position based on the relative inductor signal strength of each coil of the inductive sense array 107. More specifically, the presence of the stylus 150 changes the individual inductor currents for each coil in the inductive sense array 107 based on their relative proximity to the stylus. The maximum signal strength approximates the stylus 150 touch position on the accompanying touch screen.
The host CPU 120 sends a high frequency carrier signal to the stylus 150 via an amplifier (not shown), a transmitter 130, an impedance matching circuit 135, and the inductive sense array 107. The stylus 150 receives and utilizes the high frequency carrier signal for self-powering and data transmission. In operation, the stylus 150 rectifies (rectifier 175) and regulates (power regulator 180) the carrier signal and feeds the resultant signal to the MCU 155 and driver 160. The MCU 155 measures force sensors (force tip 190 and force eraser 195) and button states (button(s) 185) and couples the resultant data signal to the driver 160. The driver 160 drives the inductor tip 170 and inductor eraser 165, which inductively couples the stylus 150 to the inductive sense array 107.
Stylus 150 sensing is implemented largely independent of the finger-sensing capability of the touch screen. As described above, stylus tracking requires generating an alternative current (AC) signal by the inductive sense array 107 and inductively coupling the AC signal to the tip of the stylus 150. The inductive sense array 107, located behind the touch screen, in turn receives the stylus signal and the Host CPU 120 interpolates the position of the stylus tip (inductor tip 170) based on the relative magnitude of the received stylus signals at each of the inductive sensors of the inductive sense array 107. While inductive sensing may be reliable, inductive stylus tracking solutions exhibit serious commercial disadvantages including high power consumption, high electromagnetic interference (“EMI”), high manufacturing costs, and heavy construction. Furthermore, retro fitting an existing touch sensor (passive touch object sensor) to include independent stylus tracking would require an additional PCB 105 layer to incorporate the inductive sense array 107.
FIG. 2A is a block diagram illustrating a conventional embodiment of a host device 200 for tracking the position of a touch object on a radio frequency (“RF”) sense array. The host device 200 includes an Indium-Tin-Oxide (“ITO”) panel 205, a receiver 210, a data decoder 215, a host CPU 220, and a PC 225. In FIG. 2B, the stylus 250 includes a force sensor 255, a measurer 260, a modulator 265, an amplifier 270, a stylus tip 275, and a reference clock 280. The stylus 250 is typically battery powered (not illustrated).
In operation, the stylus 250 generates, amplifies, and couples an RF carrier signal from the stylus tip 275 to the ITO panel 205 via RF coupling. The ITO panel 205 functions as an antenna and receives the RF carrier signal from the stylus 250 as described below with respect to FIG. 2B. The selective receiver 210 demodulates the RF carrier signal and couples a touch position signal to the host CPU 220 and a force data signal to the data decoder 215. The data decoder 215 extracts the force data and couples it to the host CPU 220. The host CPU 220 calculates the stylus touch position based on the relative maximum amplitude of the RF signal detected on the ITO lines of ITO panel 205. The host CPU 220 further determines the force applied to the stylus based on the force data. The PC 225 controls the host CPU 220.
FIG. 2B is a block diagram illustrating a conventional embodiment of an active stylus used in a system for tracking the position of a touch object on an RF sense array. The measurer 260 of stylus 250 measures the force induced on the force sensor 255 and the modulator 265 modulates the resultant force data with a carrier frequency, provided by the reference clock 280. The amplifier 270 amplifies the modulated signal and transmits the modulated carrier frequency from the stylus tip 275. As described above, the host 200 decodes the modulated carrier signal and transmits the result to the PC 225. While an RF sense array solution may offer cost savings and a reduced component count, they require special narrow band receivers on the host 200 and are subject to RF noise and interference. Consequently, conventional touch panel solutions may have significant disadvantages in cost, performance, applicability, and reliability.